The present invention relates to a gas turbine system utilizing coal fuel. More specifically, the present invention relates to a coal fuel gas turbine system in which the amount of compressed air directed to the gasifier is maximized by using the hot gas from the gasifier, after cooling, to cool the turbine section of the gas turbine.
The high efficiency, low capital cost and short lead time of gas turbine based power plants make them particularly attractive to electric utilities as a means of producing electrical power. Unfortunately, traditionally, gas turbines have been limited to operation on expensive, sometimes scarce, fuels--chiefly, distillate oil and natural gas. As a result of the ready availability and low cost of coal, considerable effort has been expended toward developing a gas turbine system for generating electrical power that can utilize coal as its primary fuel.
In one approach, referred to as an integrated combined gasification power plant, a portion of the compressed air from the gas turbine compressor is used to fluidize coal in a pressurized coal fluidized bed ("PCFB") gasifier so as to produce a hot, low to medium heating value gas. This hot gas is used to generate steam in a heat recovery steam generator ("HRSG"). The steam is then expanded in a steam turbine, thereby producing shaft power.
The gas discharged from the HRSG is then expanded in a turbine, thereby producing additional shaft power. In order to realize maximum power output during expansion in the turbine, the gas exiting the HRSG can be further heated in a topping combustor prior to expansion in the turbine.
Since the components of the turbine section are exposed to the hot gas from the topping combustor, they must be adequately cooled so that their temperature is maintained within allowable limits. Traditionally, this cooling is achieved by flowing relatively cool air over or within the turbine components. Since such cooling air must be pressurized to be effective, it is common practice to bleed a portion of the compressed air flowing through, or discharging from, the compressor section and divert it to the turbine components for cooling purposes. The cooling air typically enters the cylinder enclosing the turbine and flows through one or more rows of stationary vanes and thence into the cavity between the turbine discs disposed on either side of the vanes in that particular row. Generally, the air is extracted from several stages within the compressor section, the air from each compressor stage being directed to a different stage of the turbine, with the higher pressure air from the compressor going to the higher pressure stages in the turbine and lower pressure air used to cool lower pressure turbine stages. Additional compressed air is used to cool the turbine rotor, for example, by flowing cooling air through small passages formed in the rotating blades.
Unfortunately, this turbine cooling approach robs compressed air from the gasifier and, therefor, reduces the gas mass flow through the HRSG. The reduced gas mass flow through HRSG reduces the steam generation and, therefore, the power output from the steam turbine.
Accordingly, it would be desirable to provide a coal fuel gas turbine system that maximized the compressed air directed to the gasifier.